Update "Checkout" and "Committing with a Detached HEAD" sections

I read Chinese version of this great, neat git tutorial today morning, and found that two sections in Chinese version are kind of out of date, and with a few mistakes. Therefore, I'v updated these two sections in Chinese version. As it's my first pull request on Github, appreciate any comment : )

今天早上看到这篇文章的中文版，看到有两个章节并不是最新的，且有些小错误，所以更新了一下。希望大家给出宝贵意见，谢谢 : )